The present invention relates to a projection television optical system for projecting pictures from a picture tube in a projection television receiver onto a screen.
A demand for increase in the size of the picture reproduction face of a television receiver aiming at powerful pictures which cannot be obtained on a small-sized picture plane has been intensified. Thus so-called projection television receivers have been widely brought into use. In the projection television receiver, pictures reproduced on the fluorescent face of a Braun tube (picture tube) are enlarged and projected onto a screen by a projection optical system including lenses and mirrors, pictures being obtained in a large picture plane.
As a result of various improvements heretofore applied to such projection television receivers, a great advance is found as compared with several years ago. Lenses play a very important role in this projection television receiver.
Lenses according to a proposal disclosed in U.S. Pat. No. 4,348,081 or 4,300,817 provide high luminance which is 1.0 in F number although only three sheets of lens are used. This is achieved by the design utilizing at its maximum the fact that the plastic lens can be easily formed to a nonspherical shape.
It is no exaggeration to say that the projection television receiver is greatly developed owing to the lens disclosed in the aforementioned U.S. Patents. Under the existing circumstances, however, the projection television receivers are still inferior to the direct-view television receivers in picture quality, compactness and price.
Respective problems will now be described.
(1) Picture quality: Important picture quality terms of the projection television receiver are focus, brightness and contrast. The focus of a lens can be generally estimated by using the MTF (modulation transfer function). As a result of estimating the lens described in the aforementioned U.S. Pat. No. 4,348,081, the focus was very fine at the central portion of the picture plane. In the actual Braun tube, however, the luminous spectrum characteristics of the fluorescent substance have a discrete distribution. By taking this into consideration, the MTF is worsened. Especially in the low frequency region, the MTF of the lens is inferior to that of the glass lens generally used.
This is caused by the fact that regard to improvement of the color aberration is not paid in the lens according to the proposal described in the aforementioned U.S. Patent. If the color aberration was improved, the MTF would be largely improved.
Since all of the lenses according to the aforementioned proposal are composed of plastic lens, a change in temperature causes a change in refractive index, resulting in focus deterioration.
In order to increase the input power of the projection tube and improve its brightness, so-called liquid-cooled Braun tubes are recently used. In these liquid-cooled Braun tubes, the heat generated by increase in input power is cooled down. The heat is radiated from a radiation fin by the convection of the liquid to reduce and make uniform the temperature of the glass of the fluorescent face, the glass of the fluorescent face being prevented from breaking due to high temperature.
(2) Compactness: In order to achieve compactness, the number of sheets of mirror in the set has heretofore been increased. Under the existing circumstances, however, the number of sheets in use has already reached its limit. For achieving further compactness, therefore, the projection distance and the lens length must be shortened. At the same time, the mirror arrangement must also be reconsidered.
In this connection, the projection distance and the lens length of the lens according to the proposal described in U.S. Pat. No. 4,348,081 are respectively 1,200 to 1,300 mm and approximately 200 mm when a screen of 45 inches is used and the magnification is 9 to 10.
(3) Price: As described above, the conventional projection television set has problems such as a complicated mirror arrangement, large lens dimensions, use of a liquid-cooled Braun tube, and a complicated coupling structure between the Braun tube and the concave lens. As a result, the high cost is inevitable.